UniFFI generation doesn't present Swift6 compatible code for the FFI layer

[heckj]

Working on enabling Swift 6 compilation compatibility (#199) and ran into the issue where UniFFI generates a global variable as var instead of let, allowing it to be variable, which is a failure in Swift 6 compilation mode. The upstream UniFFI issue for tracking a more complete Swift 6 solution is mozilla/uniffi-rs#2279, but for this immediate need I've opened mozilla/uniffi-rs#2290, effectively a fix for this need, and at least a partial fix for UniFFI.

This issue exists to track the version of UniFFI used - and upgrade when the newer version is available that includes the fix for a non-mutable global variable. (something later than v0.28.2 of UniFFI)